1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery which comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, and has battery characteristics improved to a large degree, and also relates to a method of manufacturing the same.
2. Description of the Related Art
Recent development activities on secondary batteries are directed to reduced-weight and larger energy density intended for use as a power source for electronic appliances such as a laptop personal computer, a mobile phone and a camera-integrated VTR (video tape recorder). One known secondary battery having a large energy density is lithium ion secondary battery which typically has an energy density larger than those of a lead battery, a nickel-cadmium battery, a nickel-hydrogen battery or the like.
The lithium ion secondary battery uses, for a negative electrode active material thereof, a carbonaceous material such as non-graphitizable carbon, graphite or the like in order to obtain a relatively large capacity and excellent cycle characteristics. As for the negative electrode active material for ensuring large capacity, Japanese Patent Application Publication Laid-Open No. Hei 8-315825 discloses that appropriate selection of a starting source material for the carbonaceous material and proper control of production conditions for the carbonaceous material can further increase the capacity.
It is, however, intrinsically difficult for the above-described lithium ion secondary battery to further increase the capacity because critical capacity of the carbonaceous material used for the negative electrode active material is limited as high as 370 mAh/g or around.
One possible solution for this problem relates to a lithium ion secondary battery, in which a special kind of lithium alloy, having still some room for upgrading in capacity, is used as a negative electrode active material in place of a carbonaceous material, where the charge/discharge is based on reversible electrochemical generation/decomposition of the lithium alloy.
As for use of the lithium alloy as the negative electrode active material, use of a Li—Al alloy, a Li—Si alloy and so forth has already been known. In particular, U.S. Pat. No. 4,950,566 describes use of a Li—Si alloy for the negative electrode.
The lithium ion secondary battery using a lithium alloy for the negative electrode, however, suffers from a problem that the lithium alloy considerably expands or shrinks during the charge/discharge cycle, and the repetitive charge/discharge cycles cause repetitive expansion/shrinkage of the lithium alloy, which may pulverize the lithium alloy, may disconnect mutual contact among grains of the negative electrode active material, may degrade conductivity of the negative electrode, and thus may ruin battery characteristics.
One known measure proposed for solving this problem is such that the surface of the negative electrode active material is covered, for example, with a highly-conductive material so as to keep conductivity among grains of the lithium alloy even after the alloy is pulverized due to repetitive charge/discharge. More specifically, methods of covering the surface of the negative electrode active material with a highly conductive material using techniques such as immersing a lithium alloy into an organic solvent dissolved with a highly-conductive material, or based on a mechano-chemical reaction such as hybridization, are disclosed for example in Japanese Patent Application Publication Laid-Open Nos. 2000-173669, 2000-173670 and 2001-68096.
It is, however, still difficult for these proposals to fully suppress the degradation of the large-capacity negative electrode active material such as lithium alloy under repetitive charge/discharge cycles, and the negative electrode active materials still cannot fully exhibit their potential for higher capacity at present.